1. Field of the Invention
This invention relates in general to subscriber circuits for digital time division multiplex telecommunication installations and in particular to PCM local exchanges.
2. Description of the Prior Art
The advantages of digital transmission and exchange systems are generally known. The progress of micro-electronics makes the possibility of a full digitalization of the exchange system appear to be possible in the future wherein transmission and exchange technology will allow the conversion of analog message signals directly to digital in the subscriber terminal equipment. At the present time, however, such full digitalized systems are too expensive. Since there are many analog systems in operation, the partial digital systems will be introduced in the forseeable future. For such systems, the existing subscriber lines and subscriber terminal equipment is operated in an analog fashion and remains unchanged. The digitalization of the analog message and control signals occur in these partially digitalized systems at the output locations of the individual subscriber lines that is at the subscriber circuits. The subscriber circuits thus form the interfaces between the analog and digital portions of the system.
The problem exists of designing the circuit carriers for the subscriber circuits which are in general designed as plugable logic cards so as to obtain comparatively large "packing density" of the components as well as the necessary testing and balancing work required for the analog circuit parts which systems can be constructed with reasonable cost. Up until the present time, it has been attempted by the use of skilled use of conductor plate disc of the logic cards to attain a satisfactory compromise the demands which contradict each other in part. Particular attention is devoted to the thermal problems which occur due to the high packing density.